In the field of card-type mobile wireless antenna having a feeding point, a matching circuit, and a λ/4-antenna element mounted on a board with a wireless circuit and used by being inserted into a terminal device such as a personal computer or a PDA, an antenna has been conventionally proposed that includes a grounding conductor having a first end open-ended near the λ/4-antenna element and a second end grounded at GND so that a casing current can be reduced (refer to, for example, a patent literature 1).
According to this type of antenna, since a λ/2 current distribution can be formed without a circuit GND where a noise source exists, a dipole antenna can be constructed with a portion of the circuit GND so as to reduce the effect of noise.
There are various types of vehicle-mounted antennas including a keyless entry system in which a receiver is generally installed in a vehicle compartment. An electric field antenna such as a dipole or monopole is used as a receiving antenna.
However, in a vehicle compartment, a standing wave is formed by multipath incoming wave so that electric field can have peaks and troughs. Therefore, if the receiving antenna is located at a position corresponding to the trough of the electric field, what is called a Null state occurs, and communication performance is seriously deteriorated.
Although a space diversity or polarization diversity is generally used as a method to improve such a Null state, a receiver increases in size in either method.
A radio wave has a characteristic that when a standing wave of an electric magnetic field is at a trough, a standing wave of a magnetic field is at a peak. According to this characteristic, when the electric field and the magnetic field are separately received, the electric field and the magnetic field can compensate each other for Null. Further, since the electric field and the magnetic field come in combination with each other, there is no need to separate their antennas from each other. Accordingly, a receiver is expected to decrease in size.
In the prior art, however, since an electric field antenna and a magnetic field antenna are provided on the same GND, it is difficult to maintain isolation between the antennas. If the isolation between the electric field antenna and the magnetic field antenna is not maintained, the electric field antenna and the magnetic field antenna have the same directivity, so that the expected compensation effect cannot be achieved.
Ultimately, therefore, isolation is maintained by spatially separating the antennas from each other or by adding a circuit (e.g., balance/unbalance conversion circuit) to maintain isolation (refer to, for example, a patent literature 2).